[SPARK-20725][SQL] partial aggregate should behave correctly for sameResult

## What changes were proposed in this pull request?

For aggregate function with `PartialMerge` or `Final` mode, the input is aggregate buffers instead of the actual children expressions. So the actual children expressions won't affect the result, we should normalize the expr id for them.

## How was this patch tested?

a new regression test

Author: Wenchen Fan <wenchen@databricks.com>

Closes #17964 from cloud-fan/tmp.